1. Field of the Invention
A method and apparatus for monitoring the position of a vehicle or group of vehicles.
2. Related Art
Global positioning systems (GPS) are known in which a plurality of satellites in earth orbit are arranged to transmit signals such that a receiver located on the earth's surface is able, by triangulation techniques, to identify its location. Advances in such systems have resulted in them being included in small and relatively low cost equipment. As a result such technology is now in wide spread use in diverse equipment, for example for identification of the position of one or more vehicles.
British patent publication number 2 288 892 describes a system in which the position of a road vehicle is monitored by transmitting GPS positional data from a GPS system located on the vehicle to a base station. The data is transmitted using a radio link or GSM data link. Data is transmitted at closely spaced regular intervals so as to provide substantially real-time tracking of the vehicle.
With the system described in British patent publication number 2 288 892, the interval between successive transmission events must be relatively short in order to enable a remote monitoring station to obtain such real-time tracking data. Thus, a communication channel may be continuously kept open, tying up the channel, or opened and closed at short intervals, in which case the transmission time overhead of the protocol exchanges in creating the channel is significant in relation to the actual transmission time required for transmission of positional data. Such substantially constant use of a communications channel is expensive and can be inefficient e.g. in areas of poor reception.
French patent publication number 2 670 002 discloses the determination of vehicle position based on satellite signals from which vehicle position is calculated. A situation is described in which the satellite signals may be lost, possibly due to a vehicle entering a tunnel etc. Under these circumstances, subsequent vehicle positions are calculated by means of direction and distance measuring sensors. However, the document does not address the problem of facilitating the storage of a significant volume of positional data and is concerned primarily with providing current positional data to a driver.